This invention relates generally to radio frequency (r.f.) integrated circuits and more particularly to fabrication of lumped passive circuit components.
As is known in the art, resistors and capacitors are generally required in amplifier circuit applications. In particular, when an amplifier employing field effect transistors (FETS) and r.f. transmission lines is to operate over a radio frequency range of frequencies, resistors and capacitors are needed to match the impedance of the FET so that of the transmission lines. Such capacitors typically have values of capacitance between 0.1 and 4.0 picofarads (pf). Capacitors are also required to provide filtering of bias lines provided to the circuit. These capacitors are generally much larger in value than the aforementioned capacitors typically having values of capacitance in the range of 30-60 pf. When fabricating capacitors having a capacitance in the range of 0.1 to 4 pf, silicon nitride is often used as the dielectric. However, with a relatively low dielectric constant of 7, large capacitors using a very large portion of the circuit area would be required to provide capacitors having a capacitance in the range of 30-60 pf. Further, a capacitor having such a large area may in some applications negate the lumped element characteristic of the capacitor due to its relatively large size in comparison to the wavelength of r.f. signals fed thereto. Thus, other materials with higher dielectric constants are used to reduce the ratio of the capacitance of the capacitor to the area of the capacitor. One such material is tantalum oxide.
To provide a capacitor having a tantalum oxide dielectric, the tantalum oxide must be either deposited or grown. In depositing a tantalum oxide film, one problem is maintaining uniformity of the film. When growing tantalum oxide, the major problem involves providing a layer of tantalum (from which the tantalum oxide is grown) having no surface defects. Since surface defects are common, this approach generally has not been very successful in providing capacitors having tantalum oxide dielectrics. In particular, portions of the circuit where the substrate has deposited therein metallization steps from prior processing steps are particularly susceptible to formation of defects in the tantalum film. Prevention of defects is important because during growth of a tantalum oxide film, the existence of a defect may cause a short circuit between the tantalum layer and the substrate, and thus causing damage to the substrate.